Icehouse Climate Research Articles

Tectonics, climate, and the rise and demise of continental aquatic species richness hotspots

Continental aquatic species richness hotspots are unevenly distributed across the planet. In present-day Europe, only two centers of biodiversity exist (Lake Ohrid on the Balkans and the Caspian Sea). During the Neogene, a wide variety of hotspots developed in a series of long-lived lakes. The mechanisms underlying the presence of richness hotspots in different geological periods have not been properly examined thus far. Based on Miocene to Recent gastropod distributions, we show that the existence and evolution of such hotspots in inland-water systems are tightly linked to the geodynamic history of the European continent. Both past and present hotspots are related to the formation and persistence of long-lived lake systems in geological basins or to isolation of existing inland basins and embayments from the marine realm. The faunal evolution within hotspots highly depends on warm climates and surface area. During the Quaternary icehouse climate and extensive glaciations, limnic biodiversity sustained a severe decline across the continent and most former hotspots disappeared. The Recent gastropod distribution is mainly a geologically young pattern formed after the Last Glacial Maximum (19 ky) and subsequent formation of postglacial lakes. The major hotspots today are related to long-lived lakes in preglacially formed, permanently subsiding geological basins.

Synchronizing terrestrial and marine records of environmental change across the Eocene–Oligocene transition

Records of terrestrial environmental change indicate that continental cooling and/or aridification may have predated the greenhouse–icehouse climate shift at the Eocene–Oligocene transition (EOT) by ca. 600 kyr. In North America, marine-terrestrial environmental change asynchronicity is inferred from a direct comparison between the astronomically tuned marine EOT record and published 40Ar/39Ar geochronology of volcanic tuffs from the White River Group (WRG) sampled at Flagstaff Rim (Wyoming) and Toadstool Geologic Park (Nebraska), which are type sections for the Chadronian and Orellan North American Land Mammal Ages. We present a new age-model for the WRG, underpinned by high-precision 206Pb/238U zircon dates from 15 volcanic tuffs, including six tuffs previously dated using the 40Ar/39Ar technique. Weighted mean zircon 206Pb/238U dates from this study are up to 1.0 Myr younger than published anorthoclase and biotite 40Ar/39Ar data (calibrated relative to Fish Canyon sanidine at 28.201 Ma). Giving consideration to the complexities, strengths, and limitations associated with both the 40Ar/39Ar and 206Pb/238U datasets, our interpretation is that the recalculated 40Ar/39Ar dates are anomalously old, and the 206Pb/238U (zircon) dates more accurately constrain deposition. 206Pb/238U calibrated age–depth models were developed in order to facilitate a robust intercomparison between marine and terrestrial archives of environmental change, and indicate that: (i) early Orellan (terrestrial) cooling recorded at Toadstool Geologic Park was synchronous with the onset of early Oligocene Antarctic glaciation and (ii) the last appearance datums of key Chadronian mammal taxa are diachronous by ca. 0.7 Myr between central Wyoming and NW Nebraska.

Fragments of Late Eocene Earth-impacting asteroids linked to disturbance of asteroid belt

The onset of Earth's present icehouse climate in the Late Eocene coincides with astronomical events of enigmatic causation. At ∼36 Ma ago the 90–100 km large Popigai and Chesapeake Bay impact structures formed within ∼10–20 ka. Enrichments of 3He in coeval sediments also indicate high fluxes of interplanetary dust to Earth for ∼2 Ma. Additionally, several medium-sized impact structures are known from the Late Eocene. Here we report from sediments in Italy the presence of abundant ordinary chondritic chromite grains (63–250 μm) associated with the ejecta from the Popigai impactor. The grains occur in the ∼40 cm interval immediately above the ejecta layer. Element analyses show that grains in the lower half of this interval have an apparent H-chondritic composition, whereas grains in the upper half are of L-chondritic origin. The grains most likely originate from the regoliths of the Popigai and the Chesapeake Bay impactors, respectively. These asteroids may have approached Earth at comparatively low speeds, and regolith was shed off from their surfaces after they passed the Roche limit. The regolith grains then settled on Earth some 100 to 1000 yrs after the respective impacts. Further neon and oxygen isotopic analyses of the grains can be used to test this hypothesis.If the Popigai and Chesapeake Bay impactors represent two different types of asteroids one can rule out previous explanations of the Late Eocene extraterrestrial signatures invoking an asteroid shower from a single parent-body breakup. Instead a multi-type asteroid shower may have been triggered by changes of planetary orbital elements. This could have happened due to chaos-related transitions in motions of the inner planets or through the interplay of chaos between the outer and inner planets. Asteroids in a region of the asteroid belt where many ordinary chondritic bodies reside, were rapidly perturbed into orbital resonances. This led to an increase in small to medium-sized collisional breakup events over a 2–5 Ma period. This would explain the simultaneous delivery of excess dust and asteroids to the inner solar system. Independent evidence for our scenario are the common cosmic-ray exposure ages in the range of ca. 33–40 Ma for recently fallen H and L chondrites.The temporal coincidence of gravity disturbances in the asteroid belt with the termination of ice-free conditions on Earth after 250 Ma is compelling. We speculate that this coincidence and a general correlation during the past 2 Ga between K–Ar breakup ages of parent bodies of the ordinary chondrites and ice ages on Earth suggest that there may exist an astronomical process that disturbs both regions of the inner asteroid belt and Earth's orbit with a potential impact on Earth's climate.

Tectono-climatic implications of Eocene Paratethys regression in the Tajik basin of central Asia

Plate tectonics and eustatic sea-level changes have fundamental effects on paleoenvironmental conditions and bio-ecological changes. The Paratethys Sea was a large marine seaway that connected the Mediterranean Neotethys Ocean with Central Asia during early Cenozoic time. Withdrawal of the Paratethys from central Asia impacted the distribution and composition of terrestrial faunas in the region and has been largely associated with changes in global sea level and climate such as cooling associated with the Eocene/Oligocene transition (EOT).Whereas the regression has been dated in the Tarim basin (China), the pattern and timing of regression in the Tajik basin, 400 km to the west, remain unresolved, precluding a test of current paleogeographic models. Here we date the Paratethys regression in Tajikistan at ca. 39 million years ago (Ma), which is several million years older than the EOT (at ca. 34 Ma) marking the greenhouse to icehouse climate transition of the Cenozoic. Our data also show a restricted, evaporitic marine environment since the middle–late Eocene and establishment of desert like environments after ca. 39 Ma. The overall stratigraphic record from the Tajik basin and southern Tien Shan points to deposition in a foreland basin setting by ca. 40 Ma in response to active tectonic growth of the Pamir–Tibet Mountains at the same time. Combined with the northwestward younging trend of the regression in the region, the Tajik basin record is consistent with northward growth of the Pamir and suggests significant tectonic control on Paratethys regression and paleoenvironmental changes in Central Asia.

A model-based evaluation of tropical climate in Pangaea during the late Palaeozoic icehouse

Abstract The late Palaeozoic ice age (LPIA) is the Earth's penultimate “icehouse climate.” Geological proxies for tropical Pangaean climate during the LPIA are significantly modulated on 100–400 kyr and sub-100 kyr scales. In addition, some geological proxies suggest that equatorial continental areas may have been colder and more arid during LPIA glacial intervals than during late Cenozoic glacial intervals. Furthermore, the relationship between polar and tropical climate variability remains controversial. The climate dynamics underlying these phenomena are only partly understood. Nevertheless, past modeling of LPIA climate has suggested that precipitation near the Equator was modulated by monsoonal circulation that itself was modulated by orbital variability. Here, the Earth's climate during Asselian–Sakmarian time (299–284 Ma) was simulated with the Community Climate System Model version 3 (CCSM3). These simulations address model climate sensitivity to direct and indirect effects of glaciation as well as variability in the Earth's orbit. The results of these simulations suggest that sea level and orbitally forced monsoon variability were the primary controls on tropical precipitation when ice sheets were confined to polar latitudes. Any impact of orbital forcing on glaciation therefore could have led to aliasing between glacial and monsoonal effects in particular regions, perhaps explaining contrasting geologically-based interpretations of how tropical climate responded to glaciation. Glaciation of the Central Pangaean Mountains would have led to widespread cold and aridity in equatorial Pangaea, but forming these glaciers would have required an unknown additional climate forcing with an unknown impact on tropical precipitation.

Early to middle Miocene vegetation history of Antarctica supports eccentricity-paced warming intervals during the Antarctic icehouse phase

Recent palynological evidence from the upper section (<664mbsf) of the Miocene ANDRILL 2A (AND-2A) core in the Ross Sea indicated a brief warm period during the Mid Miocene Climatic Optimum (MMCO), during Antarctica's icehouse phase. The nature of other Miocene climate fluctuations in Antarctica remains poorly resolved. Here, we present new palynological data from the lower section of the AND-2A core (>664mbsf) that reveal additional intervals of increased palynomorph abundance and diversity during the Antarctic early to middle Miocene. This evidence is consistent with a relatively dynamic early Miocene Antarctic icehouse climate with intervals of increased temperature and moisture, prior to the MMCO. Comparison with previous AND-2A studies indicates that periods of increased palynomorph abundance in the lower stratigraphic section largely coincide with ice sheet minima, distal ice conditions, and the presence of freshwater (i.e. increased precipitation, meltwater run-off, and freshwater ponds). Decreases in plant productivity mostly coincide with increased glaciation, decreased freshwater, and ice-proximal conditions during the Antarctic Miocene. Comparing the palynological data with other AND-2A data and global climate proxies (e.g., pCO2, δ18O, relative sea level, Milankovitch cycles) helps to resolve questions regarding the driving forces behind climate and vegetation change. We found that palynomorph assemblages reflecting generally warmer conditions are largely associated with 400-kyr eccentricity maxima, while assemblages indicative of colder conditions coincide with 400-kyr eccentricity minima. These data are consistent with other findings that indicate the early to middle Miocene climate was eccentricity-paced.

Middle Eocene to Late Oligocene Antarctic glaciation/deglaciation and Southern Ocean productivity

During the Eocene-Oligocene transition, Earth cooled significantly from a greenhouse to an icehouse climate. Nannofossil assemblages from Southern Ocean sites enable evaluation of paleoceanographic changes and, hence, of the oceanic response to Antarctic ice sheet evolution during the Eocene and Oligocene. A combination of environmental factors such as sea surface temperature and nutrient availability is recorded by the nannofossil assemblages of and can be interpreted as responses to the following changes. A cooling trend, started in the Middle Eocene, was interrupted by warming during the Middle Eocene Climatic optimum and by short cooling episodes. The cooling episode at 39.6 Ma preceded a shift toward an interval that was dominated by oligotrophic nannofossil assemblages from ~39.1 to ~36.2 Ma. We suggest that oligotrophic conditions were associated with increased water mass stratification, low nutrient contents, and high efficiency of the oceanic biological pump that, in turn, promoted sequestration of carbon from surface waters, which favored cooling. After 36.2 Ma, we document a large synchronous surface water productivity turnover with a dominant eutrophic nannofossil assemblage that was accompanied by a pronounced increase in magnetotactic bacterial abundance. This turnover reflects a response of coccolithophorids to changed nutrient inputs that was likely related to partial deglaciation of a transient Antarctic ice sheet and/or to iron delivery to the sea surface. Eutrophic conditions were maintained throughout the Oligocene, which was characterized by a nannofossil assemblage shift toward cool conditions at the Eocene-Oligocene transition. Finally, a warm nannofossil assemblage in the Late Oligocene indicates a warming phase.

Icehouse–greenhouse variations in marine denitrification

Abstract. Long-term secular variation in the isotopic composition of seawater fixed nitrogen (N) is poorly known. Here, we document variation in the N-isotopic composition of marine sediments (δ15Nsed) since 660 Ma (million years ago) in order to understand major changes in the marine N cycle through time and their relationship to first-order climate variation. During the Phanerozoic, greenhouse climate modes were characterized by low δ15Nsed (∼−2 to +2‰) and icehouse climate modes by high δ15Nsed (∼+4 to +8‰). Shifts toward higher δ15Nsed occurred rapidly during the early stages of icehouse modes, prior to the development of major continental glaciation, suggesting a potentially important role for the marine N cycle in long-term climate change. Reservoir box modeling of the marine N cycle demonstrates that secular variation in δ15Nsed was likely due to changes in the dominant locus of denitrification, with a shift in favor of sedimentary denitrification during greenhouse modes owing to higher eustatic (global sea-level) elevations and greater on-shelf burial of organic matter, and a shift in favor of water-column denitrification during icehouse modes owing to lower eustatic elevations, enhanced organic carbon sinking fluxes, and expanded oceanic oxygen-minimum zones. The results of this study provide new insights into operation of the marine N cycle, its relationship to the global carbon cycle, and its potential role in modulating climate change at multimillion-year timescales.

High‐precision U–Pb age and duration of the latest Devonian (Famennian) Hangenberg event, and its implications

AbstractPrecise U–Pb zircon dates from three volcanic ash beds that bracket the Hangenberg Shale in the Holy Cross Mountains, Poland, constrain the age and duration of one of the most significant palaeobiological events of the Palaeozoic Era, the Hangenberg Event. It is linked to a terminal Devonian global shift from greenhouse to icehouse climate conditions, a global transgression, and widespread black shale deposition. Our results constrain the Hangenberg Event to between 358.97 ± 0.11 Ma and 358.89 ± 0.20 Ma, with a calculated duration of 0.05 +0.14/−0.05 Ma. A third, underlying ash bed yielded a distinctly older age of 359.97 ± 0.46 Ma. The duration of ~50–100 ka. for the event is comparable to those of Quaternary glaciations, and is consistent with both a glacio‐eustatic origin for the eustatic fluctuations and changes in ocean chemistry that led to this major reorganization of the biosphere.

A stable and hot Turonian without glacial δ18O excursions is indicated by exquisitely preserved Tanzanian foraminifera

A shift from the icehouse climate in which humans evolved to a Late Cretaceous–like greenhouse climate is an often-repeated cautionary prediction of the consequences of continued anthropogenic CO 2 emissions. The corollary, that understanding the past might help predict the future, has justified many Late Cretaceous studies, but important questions remain about climate stability and sensitivity. New δ 18 O measurements of more than 1000 samples of exceptionally well preserved foraminifera (8 planktic and 11 benthic taxa) from two sites in Tanzania indicate that hot and remarkably stable conditions prevailed in the region during the Turonian, including during a proposed greenhouse glacial event. Planktic taxa have δ 18 O values largely between –4.0‰ and –5.0‰, suggesting surface-water temperatures between 30 and 35 °C. Estimates for seafloor temperatures are between 18 and 25 °C. No parallel shifts in δ 18 O values are observed among planktic and benthic taxa, contradicting an often-cited line of evidence for greenhouse glaciations and supporting an effectively ice-free Turonian world.

New Schizolepis Fossils from the Early Cretaceous in Inner Mongolia, China and its Phylogenetic Position

Abstract:Three Schizolepis species collected from the Lower Cretaceous layer of the Huolinhe Basin, Inner Mongolia, China are described. These fossils are Schizolepis longipetiolus Xu XH et Sun BN sp. nov., which is a new species, Schizolepis cf. heilongjiangensis Zheng et Zhang, and Schizolepis neimengensis Deng. The new species is a well‐preserved female cone, slender and cylindrical in shape. The seed–scale complexes have long petioles and are arranged on the cone axis loosely and helically. The seed scales are divided into two lobes from the base. Each lobe is semicircular or elongate ligulate in shape, widest at the middle or the lower middle part, with an obtuse or bluntly pointed apex. The inner margin is almost straight and the outer margin is strongly arched. On the surface of the lobe, there are longitudinal and somewhat radial striations from the base to the margin. The seed is borne on the adaxial surface at the base or middle of each lobe. Schizolepis was established in 1847, and, although more than twenty species have been discovered and reported, its phylogenetic position is controversial because of the imperfection of fossils. Most authors have considered there to be a close evolutionary relationship between Schizolepis and extant Pinaceae. Here, we analyze characteristics and compare Schizolepis with Picea crassifolia Kom, which is morphologically most similar to Schizolepis. The results indicate that the genus probably has a distant evolutionary relationship with extant Pinaceae. A detailed statistical analysis of the global paleogeographic distribution of Schizolepis showed that all the fossils of this genus appeared in strata ranging from the Upper Triassic to the Lower Cretaceous in the North Hemisphere, being rare in the Upper Triassic and Lower Jurassic, but being very common from the Middle Jurassic to the Lower Cretaceous, and particularly abundant in the Lower Cretaceous. According to the statistical results, we speculate that the genus originated in Europe in the Late Triassic then spread from Europe to Asia between the Late Triassic and the Late Jurassic. In the Early Cretaceous most species existed in China's three northeastern Provinces and the Inner Mongolia Autonomous Region and adjacent areas. Combining the paleogeographic distribution of the genus with ancient climatic factors, we deduced that Schizolepis began to decline and became extinct in the Early Cretaceous, and the reason for its extinction is closely related to the icehouse climate during the Early Cretaceous.

Oxygen and hydrogen isotope compositions of paleosol phyllosilicates: Differential burial histories and determination of Middle–Late Pennsylvanian low-latitude terrestrial paleotemperatures

The clay mineralogy, chemistry, and stable hydrogen and oxygen-isotope compositions were measured from 20 phyllosilicate samples representing 11 Pennsylvanian-age paleosol profiles taken from three cores in the Illinois basin in order to assess their utility as proxies of low-latitude terrestrial paleotemperatures. The majority of the samples are mineralogical mixtures of illite–smectite (I/S), kaolinite, and rarely discrete illite. Samples from a shallowly-buried locality in the northern part of the basin are dominantly composed of smectite-rich I/S, with variable amounts of kaolinite, and no discrete illite. Samples from deeply-buried, interior parts of the basin are composed of illite-rich I/S, variable amounts of kaolinite, and discrete illite.These phyllosilicate mixtures have δ18OV-SMOW and δDV-SMOW values that range from 17.2‰ to 23.0‰ and −56‰ to −27‰, respectively. Assuming that the phyllosilicates preserve a record of isotopic equilibrium with Pennsylvanian meteoric waters, these oxygen and hydrogen isotope values correspond to crystallization temperatures ranging from 22±3°C to 55±3°C. The clay mineralogy, phyllosilicate δ18O and δD values and calculated crystallization temperatures of 44°C to 55°C from deeply buried localities in the interior of the basin are not consistent with a pedogenic origin. Instead, these trends are considered to be the result of diagenetic recrystallization of pedogenic minerals in response to greater depths of burial (by ~1.5 to 3km) in the southerly, basin-center localities, as well as an interval of middle Permian elevated heat flow associated with magmatic intrusions in the southern part of the basin.Phyllosilicate mineralogy, δ18O and δD values, and calculated phyllosilicate crystallization temperatures from a shallowly buried, northern locality in the Illinois basin are consistent with a pedogenic origin, and reveal a long-term warming trend from an average temperature of 23±3°C in the lower Desmoinesian to an average temperature of 32±3°C in the Missourian. This temperature change is coincident with a significant change in the composition of wetland vegetation in Euramerica, which has been attributed to a shift in low-latitude Pennsylvanian climate towards warmer and drier conditions in the Late Pennsylvanian. This study reveals the presence of a dynamic Late Paleozoic paleoequatorial icehouse climate characterized by significant low-latitude temperature variability unprecedented on modern Earth.

Soil formation in the Transantarctic Mountains from the Middle Paleozoic to the Anthropocene

In the Transantarctic Mountains (TAMs), soils from the middle Paleozoic and from the Oligocene to the present have been examined. Soils representing other sections of the geologic column are missing, probably because of the low proportion of ice-free areas (0.35%) in Antarctica. The evolution of soils in Antarctica reflects changes in climate and geologic conditions as the continent became separated and increasingly isolated from Gondwana. A greenhouse climate existed during the middle Paleozoic; and an icehouse climate began in the early Oligocene. The climate of the TAMs has become increasingly hyper-arid since the middle Miocene. Humans have had a dramatic effect on the climate of the TAMs during the Anthropocene. The first forest soils (under Callixylon–Archaeopteris forest) on Earth, identified as Alfisols, were discovered in Antarctica and assigned to the middle Devonian. During the Permian, Dicroidium forests covered Entisols and Inceptisols. In the Oligocene, Nothofagus–Podocarpaceae forests contained Gelisols. Miocene-aged soils enriched in silt are common in the TAMs, but they tend to be poorly developed because they have been eroded. A soil evolutionary sequence exists in the TAMs from the Holocene through the Pliocene that includes Glacic Haploturbels on ice-cored Holocene drift, Typic Haploturbels on late Pleistocene surfaces, Typic Anhyorthels on middle Pleistocene surfaces, Salic Anhyorthels on early Pleistocene surfaces, and Petrosalic Anhyorthels on Pliocene surfaces. These changes reflect gradual sublimation of ice in ice-cored drift and ice-wedge polygons, a recovery of the surface from cryoturbation, accumulation of salts, and eventual development of a salt pan. Soils of the TAMs are undergoing rapid change as the climate warms, including loss of semi-permanent snowbanks, an expansion of the hyporheic zone, flushing of salts from soils along valley walls, and the development of thermokarst.

Sequence Architecture of the Bighorn Dolomite, Wyoming, USA: Transition To the Late Ordovician Icehouse

Abstract Although major Late Ordovician glaciation was confined to the terminal Ordovician Hirnantian Stage, lasting less than 2 million years, recent isotopic evidence suggests a prolonged interval of slow growth in polar ice caps. Tropical carbonate platforms from passive margins and epicontinental seas offer the opportunity to monitor changes in eustatic sea-level changes that accompanied this transition from a greenhouse towards an icehouse climate. The Late Ordovician Bighorn Dolomite of northern Wyoming and adjacent states preserves such a transition. The Bighorn Dolomite was deposited on a northeastward-facing, subequatorial platform, and it consists of three third-order depositional sequences. Each is bounded by a regional erosional surface, commonly with erosional truncation visible at outcrop scale and commonly mantled by a distinctive brecciated horizon. The Bighorn Dolomite is dominated by highly bioturbated and fossiliferous open marine shallow subtidal facies, but portions also contain dolomitic bioturbated restricted marine shallow subtidal and laminated peritidal facies, as well as thin-bedded argillaceous and fossiliferous deep subtidal limestone. The three depositional sequences preserve a transition in their internal cyclicity that is consistent with computer modeling of the stratigraphic signal of increasing period and amplitude of eustatic sea-level changes. The lowermost sequence, which spans the Lander and Steamboat Point members, consists of homogeneous, acyclic, shallow subtidal facies, with a few meter-scale peritidal parasequences near its top. The middle sequence, which corresponds to the Leigh Member, contains acyclic to weakly cyclic restricted shallow subtidal and peritidal facies. The uppermost sequence, consisting of the Horseshoe Mountain Member, is dominated by five decameter-scale parasequences composed of open marine shallow subtidal, restricted shallow subtidal, and peritidal facies. Overall, the Bighorn Dolomite is interpreted to represent deposition during a shift from low-amplitude (a few meters), short-period (~ 20 kyr) cyclicity to moderate-amplitude (~ 10 m), long-period (100 kyr) cyclicity indicating a shift from greenhouse conditions towards icehouse conditions, a shift that agrees with recent isotopic evidence for changing Late Ordovician climate.

100 Million Years of Reef Prosperity and Collapse: Ordovician to Devonian Interval

From the beginning of the Late Ordovician (Sandbian: 460.9myr) through end Devonian (Famennian: 359.2myr), coral-stromatoporoid sponge reefs formed a remarkable, evolving ecosystem that dominated sediment production on tropical carbonate platforms in a calcitic ocean. This was a time of maximal and unparalleled reef development in the Phanerozoic, with reef tracts vastly exceeding in size and biodiversity of those in the Holocene (e.g., the Great Barrier Reef). Within this circum-equatorial niche, the calcitic tabulate and rugose corals, and the aragonitic (or high Mg calcite) stromatoporoid sponges, were the primary Middle Paleozoic reef frame builders. These were supplemented ecologically and skeletally by now extinct groups of calcitic bryozoans, crinoids, brachiopods, and red algae, alongside aragonitic green algae, and enigmatic CaCO3precipitating and binding calcimicrobes. This 100 myr long Middle Paleozoic reef consortium thrived under SST averages of 30°+, to latitudes as high as 45°–55°, under high atmospheric CO2conditions of 6000+ ppm, and sealevels 150–200 m higher than today. This reef ecosystem was disrupted by several relatively short duration south polar glacial episodes, centered around northern Gondwana, defining the O/S boundary Mass Extinction Events (MEEs). Nearly all coral and stromatoporoid families survived this MEE: there were losses at the genus level. Reef-building stopped nearly everywhere, and during the ‘recovery’ interval, solitary rugose corals initially prevailed, and stromatoporoids were small. Full global re-establishment of the reef ecosystem, and biodiversity, took another 3–4 million years (not until the late Aeronian, Early Silurian). This was followed by a remarkable reef expansion in the Middle Silurian (Wenlock), then by declines in the latest Silurian (Ludlow-Pridoli), and earliest Devonian (Lochkovian) possibly due to sealevel lowstands, tectonic plate re-assembly, and ocean current re-direction. Maximal Phanerozoic reef success was during the Emsian-Givetian, when some 15 barrier reef tracts more than 1100 km long flourished in tropical shallow seas. Reef-building coral diversity exceeded 200 genera, and the calcifying stromatoporoids evolved 60+ genera, especially in the ‘Old World’ faunal province (Euramerica, Cathaysia, northern Australia). Near the end of the Middle Devonian (mid- to late Givetian), the primary reef dwellers declined sharply in diversity, marked generally by sealevel lowstand, followed in the Frasnian (Late Devonian) by shrinking latitudes for carbonate platforms, and reduced reef accommodation space. Sharp cooling, with the arrival of a global Icehouse climate, and aragonitic oceans, led to the second largest Phanerozoic Mass extinction around the Frasnian/Famennian boundary, with reef builder and reef inhabitant losses exceeding those of the O/S MEE. The global absence of coral-sponge reefs persisted for nearly all of the 16 myr long Famennian, as total CaCO3production fell some 60–90%, as aragonitic oceans took over. Only small and scattered Famennian coral-stromatoporoid patch reefs are known, with the last of these in the late Famennian (Strunian), punctuated by total disappearance of the whole keystone reef-building order. Famennian and Strunian corals belonged to Carboniferous families. During the Famennian, calcimicrobes, the first calcifying foraminiferans, and select ‘lithistid’ calcareous sponges dominated a highly stressed reef ecosystem, lacking barrier reef tracts. Biodiversity and reef construction were decoupled under global climatic stress during the succeeding icehouse Late Paleozoic.

Spatial and Temporal Evolution of a Permian Submarine Slope Channel-Levee System, Karoo Basin, South Africa

Abstract The Laingsburg depocenter of the SW Karoo Basin, South Africa, includes a series of sandstone-dominated deepwater clastic deposits (units A and B of the Laingsburg Formation and Unit C to Unit G of the overlying Fort Brown Formation) separated vertically by regional mudstones and records basin-floor to upper-slope deposition during the Permian icehouse climate. Unit C provides nearly continuous exposures over tens of kilometers, and the presence of regionally persistent internal mudstone markers (lower and upper C mudstones) allows the distribution of sedimentary facies and architectural elements of a slope-turbidite system to be documented for more than 30 km downslope and 20 km across slope. The spatial and temporal distribution of architectural elements and interpreted depositional environments (external levees, channel belts confined by a combination of basal erosion and overbank aggradation along the margins, and distributive frontal splays) reveals distinct changes in the sedimentation pattern and stratal architecture of the turbidite system through time. Unit C evolved in a stepwise manner from a weakly incised, levee-confined channel belt and its downdip distributive frontal splays (C1), through a more entrenched and sinuous channel–levee complex set that fed submarine fans farther into the basin (C2), to a regionally backstepping package of thin-bedded deposits of a distal distributive system (C3). Unit C is interpreted as a lowstand sequence set, composed of three depositional sequences, each of which includes a sandstone-dominated lowstand systems tract (C1, C2, and C3) and a mudstone-prone interval that is taken to represent the transgressive and highstand systems tracts. The overlying combined transgressive and highstand sequence set is marked by the 30-m-thick C–D mudstone interposed between Unit C and Unit D. Unit C and the C–D mudstone together form the Unit C composite sequence. At the scale of the Unit C lowstand sequence set, the evolutionary trend from C1 to C3 reflects an overall basinward then landward stepping of the system across the depositional profile, which is interpreted as the product of an overall waxing then waning of flow energy (volume and efficiency).

Seawater calcium isotope ratios across the Eocene-Oligocene transition

AbstractDuring the Eocene-Oligocene transition (EOT, ca. 34 Ma), Earth's climate cooled significantly from a greenhouse to an icehouse climate, while the calcite (CaCO3) compensation depth (CCD) in the Pacific Ocean increased rapidly. Fluctuations in the CCD could result from various processes that create an imbalance between calcium (Ca) sources to, and sinks from, the ocean (e.g., weathering and CaCO3 deposition), with different effects on the isotopic composition of dissolved Ca in the oceans due to differences in the Ca isotopic composition of various inputs and outputs. We used Ca isotope ratios (δ44/40Ca) of coeval pelagic marine barite and bulk carbonate to evaluate changes in the marine Ca cycle across the EOT. We show that the permanent deepening of the CCD was not accompanied by a pronounced change in seawater δ44/40Ca, whereas time intervals in the Neogene with smaller carbonate depositional changes are characterized by seawater δ44/40Ca shifts. This suggests that the response of seawater δ44/40Ca to changes in weathering fluxes and to imbalances in the oceanic alkalinity budget depends on the chemical composition of seawater. A minor and transient fluctuation in the Ca isotope ratio of bulk carbonate may reflect a change in isotopic fractionation associated with CaCO3 precipitation from seawater due to a combination of factors, including changes in temperature and/or in the assemblages of calcifying organisms.

Oxygen isotopes of East Asian dinosaurs reveal exceptionally cold Early Cretaceous climates

Early Cretaceous vertebrate assemblages from East Asia and particularly the Jehol Biota of northeastern China flourished during a period of highly debated climatic history. While the unique characters of these continental faunas have been the subject of various speculations about their biogeographic history, little attention has been paid to their possible climatic causes. Here we address this question using the oxygen isotope composition of apatite phosphate (δ ) from various reptile remains recovered from China, Thailand, and Japan. δ values indicate that cold terrestrial climates prevailed at least in this part of Asia during the Barremian-early Albian interval. Estimated mean air temperatures of about 10 ± 4 °C at midlatitudes (∼ 42 °N) correspond to present day cool temperate climatic conditions. Such low temperatures are in agreement with previous reports of cold marine temperatures during this part of the Early Cretaceous, as well as with the widespread occurrence of the temperate fossil wood genus Xenoxylon and the absence of thermophilic reptiles such as crocodilians in northeastern China. The unique character of the Jehol Biota is thus not only the result of its evolutionary and biogeographical history but is also due to rather cold local climatic conditions linked to the paleolatitudinal position of northeastern China and global icehouse climates that prevailed during this part of the Early Cretaceous.

Evidence for orbital forcing of dust accumulation during the early Paleogene greenhouse

The accumulation of wind blown (eolian) dust in deep-sea sediments reflects the aridity/humidity conditions of the continental region supplying the dust, as well as the “gustiness” of the climate system. Detailed studies of Pleistocene glacial-interglacial dust fluxes suggest changes in accumulation rates corresponding to orbital variations in solar insolation (Milankovitch cycles). While the orbital cycles found in sedimentary archives of the Pleistocene are intricately related to glacial growth and decay, similar global orbital signals recognized in deep-sea sediments of early Paleogene age, the last major greenhouse interval ∼65–45 million years ago, could not have been linked to the waxing and waning of large ice sheets. Thus orbital signals recorded in early Paleogene sediments must reflect some other climate response to changes in solar insolation. To explore the potential connection between orbital forcing and the climate processes that control dust accumulation, we generated a high-resolution dust record for ∼58 Myr old sediments from Shatsky Rise (ODP Site 1209, paleolatitude ∼15°N–20°N). The dust accumulation data provide the first evidence of a correlation between dust flux to the deep sea and orbital cyclicity during the early Paleogene, indicating dust supply responded to insolation forcing during the last major interval of greenhouse climate. Furthermore, the relative amplitude of the dust flux response during the early Paleogene greenhouse was comparable to that during icehouse climates. Thus, subtle variations in solar insolation driven by changes in Earth's orbit about the Sun may have had a similar impact on climate during intervals of overall warmth as they did during glacial-interglacial states.

Export productivity and carbonate accumulation in the Pacific Basin at the transition from a greenhouse to icehouse climate (late Eocene to early Oligocene)

[1] The late Eocene through earliest Oligocene (40–32 Ma) spans a major transition from greenhouse to icehouse climate, with net cooling and expansion of Antarctic glaciation shortly after the Eocene/Oligocene (E/O) boundary. We investigated the response of the oceanic biosphere to these changes by reconstructing barite and CaCO3 accumulation rates in sediments from the equatorial and North Pacific Ocean. These data allow us to evaluate temporal and geographical variability in export production and CaCO3 preservation. Barite accumulation rates were on average higher in the warmer late Eocene than in the colder early Oligocene, but cool periods within the Eocene were characterized by peaks in both barite and CaCO3 accumulation in the equatorial region. We infer that climatic changes not only affected deep ocean ventilation and chemistry, but also had profound effects on surface water characteristics influencing export productivity. The ratio of CaCO3 to barite accumulation rates, representing the ratio of particulate inorganic C accumulation to Corg export, increased dramatically at the E/O boundary. This suggests that long-term drawdown of atmospheric CO2 due to organic carbon deposition to the seafloor decreased, potentially offsetting decreasing pCO2 levels and associated cooling. The relatively larger increase in CaCO3 accumulation compared to export production at the E/O suggests that the permanent deepening of the calcite compensation depth (CCD) at that time stems primarily from changes in deep water chemistry and not from increased carbonate production.